The present invention relates to films of high clarity made on blown film coextrusion equipment and especially but not exclusively to metallocene and Ziegler-Natta catalyst produced ethylene copolymers and titanium chloride derived ethylene copolymers from which the films may be made. The films may be made using modified processing conditions and/or using selected density differentials between core and outer layer or layers of the film.
The manufacture of coextruded blown film and the equipment for making it are well known. As the film is drawn from the annular die in molten form, the film solidifies and crystallizes upon cooling. Multi-layer films have been made in which at least one surface or outer layer are made to facilitate heat-sealing. A core film layer may be used to provide strength, impact resistance, stretchability, the main physical properties of the film, or combinations thereof. Layers intermediate the surface layer and core layer, also referred to as inner layers in this context, may facilitate the mutual adhesion of the layers and/or may impart barrier properties against the transmission of moisture, carbon dioxide, oxygen, other gases and the like.
The polymers that are used in such processes for packaging applications include in general polyethylene, polypropylene, ethylene vinyl alcohol, various tie layers and the like.
Usually higher clarity is achieved in blown film applications by the inclusion of softer polyolefins, such as very low density polyethylene, ethylene vinyl acetate copolymers and the like. Such materials, while offering benefits in clarity, often result in undesirably soft films or film structures.
There is a commercial need therefore for a film and/or a film processing technique that will deliver higher clarity to a blown, multilayer film without compromising the desired stiffness.
We have discovered that at least two techniques will accomplish the goal of higher clarity in blown films: (1) raising the temperature of one or more core layers in a multilayer film; (2) creating a density differential between the core and surface layer or layers, the core layer being of a higher density, or combinations of (1) and (2). This will result in blown films having substantially better clarity, manifested in part in lower haze values. These films will generally have at least three layers.
The polymers making up the blown multilayer films may be made with metallocene catalyst especially using methyl alumoxane or non-coordinating anions as activators or using conventional Ziegler-Natta catalysts such as titanium tetrachloride and an aluminum alkyl as activator or combinations of metallocene and Ziegler-Natta catalyst produced materials. The melt index can be one suitable for melt extrusion of films of from 0.1 to 3, preferably from 0.5 to 2 as measured by ASTM 1238, condition E.
The polymers may be made using known processes such as solution, slurry, gas phase and high-pressure polymerization techniques. According to process type, the catalyst components may be used on their own or on a support. Scavengers such as aluminum alkyls may be used to reduce the effects of catalyst poisons and catalyst activity in these processes.
Metallocene derived polyethylene has a lower blocking effect than Ziegler-Natta produced materials when used as one or both surface layers and will therefore generally be preferred in this application. In addition the increased clarity, lower haze film is stiffer and may display a higher yield and secant modulus, while the impact strength remains satisfactory. A smoother film surface results than is the case with prior techniques.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.
Various embodiments of our invention concern certain classes of blown multilayer polyolefin films, and their uses. These multilayer films exhibit improved optical properties over multilayer films made from generally the same or similar materials, but fabricated in a conventional manner, for example, with substantially the same melt temperature in all layers, or with the same or similar density.
A detailed description of certain techniques for making multilayer films that are within the scope of our invention, and the preferred applications of multilayer films follow.
Those skilled in the art will appreciate that numerous modifications to these preferred embodiments can be made without departing from the scope of the invention. For example, although the properties of three layer, multilayer films using polyethylenes are used to exemplify the attributes of the techniques and materials of the present invention, the disclosed films have numerous other uses and will provide the same properties when other polymers are used in multilayer films. To the extent that our description is specific, this is solely for the purpose of illustrating preferred embodiments of our invention and should not be taken as limiting our invention to these specific embodiments.
The use of subheadings in the description is intended to assist and is not intended to limit the scope of our invention in any way.
Embodiments of our invention may be achieved by extruding at least three polymer layers, generally from at least two extruders, where the core layer has a higher melt temperature than the skin layer or layers. The temperature differential between the outer or skin layers and the core layer or layers, will be at least 15xc2x0 C., preferably at least 20xc2x0 C., more preferably at least 25xc2x0 C., even more preferably at least 30xc2x0 C., most preferably at least 40xc2x0 C.
Frost lines in blown film are generally the phenomenon that a line or band running generally in the circumference of the blown film that indicates the demarcation between the molten polymer and the frozen or solidified polymer. The films of the present invention will preferably display two frost lines.
In another embodiment of our invention, different (lower) densities for at least the skin or outer layer or layers and the core layer or layers (higher densities than skin layer or layers) will also generally result in higher clarity than for two polyethylenes of the same or similar densities. The density differential should be at least 0.005 g/cm3, preferably 0.010 g/cm3, more preferably 0.015 g/cm3, more preferably 0.020 g/cm3, even more preferably at least 0.030 g/cm3, most preferably at least 0.040 g/cm3.
It will be understood by those of ordinary skill in the art that the techniques of differentiated density and/or differentiated temperature can be used in conjunction with one another. The effects of these two techniques may be additive, that is a 20xc2x0 C. difference in melt temperature, used in conjunction with a 0.020 g/cm3 density difference may, for instance, achieve the clarity goals sought. Other such combinations are contemplated, as long as the sought after clarity is obtained.
The ratio of thickness of the core layer and skin layers is generally preferred to be in the range of 1/1/1-1/4/1, more preferably 1/1.5/1-1/3/1, even more preferably 1/1.5/1-1/2.5/1, most preferably the ratio of core layer to skin layers will be 1/2/1, where the first and third numbers are understood to represent the skin layers and the middle number is representative of the core layer.
It will be further understood that the skin layers can be more than one polymer, for example, one skin layer may be a Ziegler-Natta produced polyethylene, while the other skin layer may be a metallocene produced polyethylene. The core layer may be a third type of polymer, for example, high density polyethylene (HDPE), but may also be identical to one of the skin layers, as long as either a density differential and/or a temperature differential sufficient to improve clarity can be maintained.
The clarity of multilayer blown films of our invention will be at least 10% improved, as measured by haze (as determined by ASTM D-1003), over the clarity or haze of films made not using techniques disclosed herein. Preferably the haze will be at least 20 percent lower, more preferably at least 40% lower, even more preferably 60% lower, most preferably 75% lower than multilayer blown films not using the techniques disclosed herein.
In embodiments of our invention, we contemplate that many polyolefins will be useful. Included, but not so limited are polyethylenes, polypropylenes, ethylene vinyl acetate, ethylene methyl acrylate, ethylene butyl acrylate, ethylene acrylic acid, ethylene methacrylic acid, and ionomers of these acids. Polyethylenes contemplated include low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE) and very low density polyethylene (VLDPE). Polyethylenes include both those made via a Ziegler-Natta catalysis, metallocene catalysis, and combinations thereof.
Polypropylenes contemplated include both those made via Ziegler-Natta and metallocene catalysis, as well as combinations thereof.
Those of ordinary skill in the art will appreciate that while structures exemplified include metallocene polyethylene and Ziegler-Natta produced polyethylenes, each in different layers, these polyethylenes can be blended in any layer. Further, while ABA or ABC structures are exemplified, (where A, B and/or C denote different polymers) other structures are also contemplated. For instance if a barrier polymer such as ethylene vinyl alcohol (EVOH) were used as the core layer, additional layers would likely be necessary. Such additional layers denoted A1 and C1, could be configured A/A1/B/C1/C, or A/A1/B/A1/A, or A/A1/B/A1/C or similar types of combinations and permutations. Also contemplated are blends of polymers in one, some or all of the layers.
Films of the present invention may be used in various packaging applications. Among these packaging applications is use as bread bags.